1. Field of the Invention
This invention relates to liquid electrophotography, especially an apparatus and method for removing carrier liquid from a photoreceptor surface or from a liquid developed toned image on a photoreceptor.
2. Background of the Art
Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoreceptor, imagewise exposing the photoreceptor to activating electromagnetic radiation (also referred to herein as “light” and including infrared, visible light and ultraviolet radiation) and thereby dissipating the charge in the exposed areas to form an electrostatic latent image, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoreceptor to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. The direct or intermediate transfer typically occurs by one of two methods: electrostatic assist (electrostatic transfer) or elastomeric assist (adhesive transfer). “Adhesive transfer” means that transfer was primarily effected by surface tension phenomena (e.g., including tack) between the receptor surface and the temporary carrier surface or medium for the toner. “Electrostatic transfer” means that transfer was primarily effected by electrostatic charges or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner.
The effectiveness of adhesive transfer is controlled by several variables including surface energy, temperature, and pressure. Electrostatic transfer is also affected by surface energy, temperature, and pressure, but the primary driving force causing the toner image to be transferred to the final substrate is via electrostatic forces.
The structure of a photoreceptor generally may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. All photoreceptors have a photoconductive layer which transports charge (either by an electron transfer of charge transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoreceptor is either negatively or positively charged such that when activating electromagnetic radiation strikes a region of the photoconductive layer, charge is conducted through the photoreceptor in that region to neutralize, dissipate or reduce the surface potential in the illuminated region. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and extend the service life of the photoconductive layer. Other layers, such as adhesive layers or priming layers or charge injection blocking layers are also used in some photoreceptors. A release layer may be used to facilitate transfer of the image from the photoreceptor to either the final substrate, such as paper, or to an intermediate transfer element.
Typically, a positively charged toner is attracted to those areas of the photoreceptor which retain a negative charge after the imagewise exposure, thereby forming a toner image which corresponds to the electrostatic latent image. The toner need not be positively charged, although that charge form or a neutral charge is preferable. Some toners (irrespective of their charge) may be attracted to the areas of the photoreceptor where the charge has been dissipated. The toner may be either a powdered material comprising a blend or association of polymer and colored particulates, typically carbon for a black image, or a liquid material of finely divided solids dispersed in an insulating liquid that is frequently referred to as a carrier liquid.
Generally, the carrier liquid is a hydrocarbon that has a low dielectric constant (e.g., less than 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image. Examples of such carrier liquids include NORPAR™ and ISOPAR™ solvents from Exxon Chemical Company.
Liquid toners are often preferable because they are capable of giving higher resolution images and require lower energy for image fixing than do dry toners. However, excess carrier liquid which is transferred to the photoreceptor can create a variety of problems. When either the elastomeric or adhesive transfer mechanism is being used, removal of excess carrier liquid is especially important. The excess carrier liquid can blot or stain the image or can cause smudging or streaking of the images. In addition, if excess carrier liquid is not removed, additional energy will be required at the image fixing step to volatilize the excess carrier liquid. Also, removal of the excess carrier liquid generally leads to improved image clarity and image density.
A variety of methods have been employed to remove excess carrier liquid from a developed toner image. These methods include squeegee rolls, air knives, corona discharge, vacuum removal, and absorption.
U.S. Pat. No. 5,420,675 to Thompson et al. discloses the use of a film forming roller which has a thin, outer layer which is compatible (referred to as ‘phillic’) with the carrier liquid and an inner layer which is carrier liquid-phobic and compressible. The film forming roller of that patent is maintained in contact with a single heating roller. The carrier liquid entrained in the film forming roller is removed by heating the liquid to a temperature greater than or equal to the flashpoint of the liquid.
U.S. Pat. No. 5,552,869 to Schilli et al. discloses a drying method and apparatus for electrophotography using liquid inks. The drying apparatus removes excess carrier liquid from an image produced by liquid electrophotography on a moving organophotoreceptor. The system includes a drying roller that contacts the organophotoreceptor, with an outer layer that absorbs and desorbs the carrier liquid and an inner layer having a Shore A hardness of 10 to 60 which is carrier liquid-phobic, and a heating means to increase the temperature of the drying roller to no more than 5° C. below the flash point of the carrier liquid. In one embodiment, the heating means includes two hot rollers and the system further includes a cooling means that cool the drying roller.
U.S. Pat. No. 5,736,286 to Kaneko et al. discloses the employment of a drying belt to remove carrier fluids in liquid inks.
The art teaches drying of a liquid toned image by absorbing or “drying” processes consisting of absorbing or adsorbing the excess carrier fluid from the image face, after the image is plated onto the photoreceptor and before the image is transferred to the receiving medium, by means of an absorptive polymer layer coated onto a roller, belt, or disk. Other methods of carrier fluid removal include: drying the image from the backside of the image using vacuum assistance through a semi-permeable membrane; thermally drying the receiving medium after the image has been transferred, absorbing by the drying member, of excess carrier fluid from a non-absorptive intermediate transfer belt after the image has been transferred to the receiving medium; and thermally evaporating the excess carrier fluid from an absorptive transfer belt and/or the image into the surrounding environment.
In cases of continuous printing, the drying belt or roller may become substantially saturated with carrier liquid at some point. At this point, regeneration or “renewing” the drying member is desirable because absorption of carrier fluid by the drying member may be repeated after the carrier liquid has been removed. Regeneration is usually facilitated by heat, pressure, or vacuum or a combination thereof. After regeneration is completed, the drying member is capable of absorbing more carrier fluid because the drying member remains unsaturated with the carrier fluid. The art teaches thermal regeneration of the drying member to prevent saturation; however, this use of a single belt or roller, which is subject to countless absorption and regeneration (or “desorption”) cycles, has many associated problems.
For example, continual contact of a heated regeneration element with the drying belt or roller can cause the belt or roller to heat up, resulting in hot offset of the liquid toned image to the hot surface of the drying belt or roller.
Another problem is associated with the need for constant absorption and regeneration of the drying member. A heating element is frequently used to evaporate the unwanted carrier liquid in the drying roller or belt. This evaporation step creates carrier liquid vapor that may be harmful to consumers and may be regulated by environmental standards. The harmful vapor must therefore be collected or rendered harmless creating a need for a vapor collection system or apparatus in the imaging line (a complex and usually costly system typically comprising at least a fan, collection ducts, and a condenser). The evaporated and condensed carrier is then stored in liquid form in the printer until disposal.
Another problem that occurs in carrier removal is that the repetitive use and regeneration of the same drying or absorptive belt or roller degrades the absorbent layer, introducing artifacts/contaminants to the toner image, and generally decreasing the life of the drying roller or belt. The high heat necessary to continually evaporate a non-volatile or high flashpoint solvent from the absorbent layer also has the effect of degrading the surface of the belt or roller. Over time a continuously re-used belt or roller will pick up sufficient contaminants (e.g. paper fibers, dust, toner particles, etc.) to increase the surface energy. If the surface energy of the roller or belt increases, it will begin to adhere to surfaces that have a lower surface energy, like the photoreceptor, the intermediate transfer member, or even the toner. To keep contaminants from altering the surface energy of the roller or belt, a cleaning mechanism is frequently employed in an attempt to maintain integrity.
An irreversible problem associated with the drying rollers and belts of the prior art is when ozone from the corona in an electrophotographic printer oxidizes the surface of the roller or belt. Once ozone damage is done, there is no possibility for renewal.
The drying rollers of the prior art are expensive to make and difficult to exchange. They frequently have a metal core, adding to the cost of manufacture. Both belts and rollers are also consumable components of a printer that generally require a visit by a service person for exchange.
These and other problems associated with drying carrier liquid from a photoreceptor and/or drying carrier liquid from a liquid developed image on a photoreceptor are known to those skilled in the art. The art continually searches for solutions to these problems and improved drying methods.